Semiconductor lasers, including distributed feedback (DFB) semiconductor lasers and external-cavity semiconductor lasers, are important laser source used in scientific research and industry.
Conventional external-cavity semiconductor lasers are usually implemented in Littrow configuration and in Littman or grazing incidence configuration, which are shown in FIGS. 1 and 2, respectively. FIG. 1 is a schematic diagram showing the structure of an external-cavity semiconductor laser with Littrow configuration, wherein laser beams emitted from semiconductor laser diode 1 are converged into parallel beams after passing through aspheric collimator 3, and then imping onto grating 12 at an incidence angle θi. After first order diffraction thereon, the resulted diffracting light is fed back to laser diode 1 along the path collinear with the incident light and in the opposite propagation direction thereof, while the mirror-reflected light from grating 12 is transmitted as output of the laser. With such configuration, the angle θi of the incident light onto grating 12 is equal to the diffraction angle θd.
FIG. 2 is a schematic diagram showing the structure of an external-cavity semiconductor laser with Littman configuration. Likewise, the laser beams emitted from semiconductor laser diode 1 are converged by aspheric collimator 3 into parallel beams, and then imping onto grating 12 at a grazing incidence angle θi. The resulted light after the first order diffraction on grating 12 is reflected by reflector 201, and then radiates back to grating 12 as feedback along the path collinear with the incident light and in the opposite transmission direction thereof. After rediffraction on grating 12, the light returns into laser diode 1, while the mirror-reflected light on grating 12 is transmitted as output of the laser. As shown in this Figure, incidence angle θi of the light onto grating 12 is greater than the diffraction angle θd of the light from grating 12. Although having smaller power, Littman configuration allows narrower spectrum than Littrow configuration. With unmovable grating 12, the wavelength of the output laser can be modulated by adjusting the angle of reflector 201.
Usually, the external-cavity semiconductor lasers mentioned above have output spectrum linewidth of up to several hundreds KHz or even up to several MHz, and DFB semiconductor lasers provide even wider linewidth, which is undesirable for many application situations.
Now, two methods for obtaining narrow linewidth output laser are usually used. One is optical-electronic feedback method, in which a portion of the light branched from the laser beam with wide linewidth is radiated into a separate F-P cavity, laser signal reflected by or transmitted through the F-P cavity is received and fed into an electronic feedback system, which locks the laser frequency to a certain resonance peak of the F-P cavity, and thus narrows the linewidth of the laser. Another method is optical feedback method, in which a confocal F-P cavity is arranged outside the laser for producing feedback light, so as to narrow the linewidth using narrow-spectrum light feedback at resonance peak of the F-P cavity, e.g. the resonant feedback semiconductor laser proposed by B. Dahmani, L. Hollberg and R. Drullinger.
F-P cavity is an important element in optical or laser research. Folded F-P cavity, in which the reflected light in a direction opposite to the incident light at folded positions has a spectrum structure different from the straight F-P cavity, can provide optical feedback with narrow linewidth. Folded F-P cavities currently available are all composed of discrete components. For example, FIG. 3 shows a folded F-P cavity composed of discrete components, which is proposed by K. Döringshoff, I. Ernsting, R.-H. Rinkleff, S. Schiller and A. Wicht. Such a folded F-P cavity (CAV) is composed of a coupler 101 and two reflectors 102 and 103, wherein coupler 101 also serves as a folded reflector. Light from coupler 101 enters the folded F-P cavity. After reflection from reflectors 102 and 103 and coupler 101 in the cavity, two reflected beams are produced, that is, a reflected beam propagating in the same direction of and collinear with mirror-reflected light of the incident light, and an other reflected light propagating in the opposite direction of and collinear with the incident light, wherein the latter can be output from the F-P cavity as light with additional function of frequency selection.
However, due to the difficulty of accurate turning of discrete components, existing folded F-P cavities composed of discrete components are sensitive to outside inferences caused by sound, mechanical vibration and temperature influence. Further, such F-P cavity usually has a relative large volume and poor system reliability.